1. Field of the Invention
This invention relates generally to an improvement in the process for carbonylating methanol to acetic acid in the presence of a rhodium-containing catalyst. More particularly, the invention relates to an improved process for regenerating the catalyst solution employed in the carbonylation reaction.
2. Description of the Prior Art
Among currently-employed processes for synthesizing acetic acid one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
An improvement in the prior-art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in copending, commonly assigned U.S. patent application Ser. No. 699,525, filed Feb. 8, 1985 and European patent application 161,874; published Nov. 21, 1985. As disclosed therein acetic acid (HOAc) is produced from methanol (MeOH) in a reaction medium comprising methyl acetate (MeOAc), methyl halide, especially methyl iodide, (MeI), and rhodium present in a catalytically-effective concentration. The invention therein resides primarily in the discovery that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4 wt. % or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14 wt. % or 15 wt. % water) by maintaining in the reaction medium, along with a catalytically-effective amount of rhodium, at least a finite concentration of water, methyl acetate and methyl iodide, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The applications teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt. %, so low that it can broadly be defined simply as "a finite concentration" of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product-recovery steps of the process wherein distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. patent application Ser. No. 699,525 is herein incorporated by reference.
In operation of the process for the carbonylation of methanol to acetic acid on a continuous basis, a solution containing the soluble catalyst complex is separated from the reactor effluent and recycled to the reactor. However, with operation over extended periods of time, corrosion products, namely, iron, nickel, molybdenum, chromium and the like form and build up in the catalyst recycle stream. Such foreign metals, if present in sufficient quantity, are known to interfere with the carbonylation reaction or accelerate competing reactions such as the water-gas reaction (carbon dioxide and hydrogen formation) and methane formation. Thus, the presence of these corrosion metal contaminants has an adverse effect on the process, in particular, a consequent loss in yield based on carbon monoxide. Further, foreign metals can react with ionic iodine thus making this component of the catalytic system unavailable for reaction with rhodium and causing instability in the catalyst system. In view of the high cost of the rhodium-containing catalyst, replacement of spent catalyst can be effected only at a prohibitive cost. Consequently, a method for regeneration of the catalyst is not only desirable but necessary.
According to U.S. Pat. No. 4,007,130, a carbonylation catalyst solution comprising the complex reaction product of a rhodium component or an iridium component, a halogen component and carbon monoxide which contains metallic corrosion products is intimately contacted with a cation exchange resin in its hydrogen form and the catalyst solution recovered free of the metallic corrosion products. As disclosed therein, the contacting is effected by passing the catalyst solution containing the undesirable corrosion metal contaminants through a bed of the cation exchange resin and recovering as the effluent from the bed the catalyst solution containing the complex rhodium or iridium component but substantially free of the corrosion products which are adsorbed on and removed by the resin bed. Upon exhaustion as indicated by breakthrough of the corrosion metal products in the effluent, the resin bed is regenerated by treatment with a mineral acid such as hydrochloric, sulfuric, phosphoric or hydriodic acid and re-used.
However, U.S. Pat. No. 4,007,130 does not contemplate using the catalyst solutions such as set forth in the aforementioned U.S. patent application Ser. No. 699,525. Thus, in the improved catalyst solutions as previously discussed, there is present a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. This additional iodide ion is present as a simple salt, and most preferably, as lithium iodide. What has been discovered is that in regenerating the catalyst solution in order to remove the metal contaminants by means of passing the catalyst solution through a bed of a cation exchange resin in the hydrogen form as disclosed in U.S. Pat. No. 4,007,130, the lithium ion in the catalyst solution is also reduced. The removal of the lithium ion from the catalyst solution greatly reduces the reactivity and stability of the reaction medium.
Accordingly, it is necessary to provide an improved process for regenerating carbonylation catalyst solutions which contain lithium to allow the removal of corrosion metal contaminants from the catalyst solutions and to avoid the removal of the desirable components from such solutions. It is therefore an object of the present invention to provide a process for treating carbonylation catalyst solutions containing lithium to remove metallic corrosion products therefrom and to recover the catalyst solution in a form suitable for return to the process as an active catalyst without the need for excessive replacement of the components therein.